1. Field
This application relates to arrangements and manifolds for stacks of solid state ion conducting electrolyte elements, and more particularly to arrangements and manifolds for assembling modular multiple stack ion conducting devices.
2. State of the Art
Solid state ion conducting devices are typically constructed from a plurality of electrolyte elements which are capable of conducting ions of a specific size or type through the element. Materials having this capability include ceramic metal oxides such as bismuth oxide and cerium oxide, polymeric electrolyte membranes, and immobilized molten electrolyte membranes. The membranes are more pliant than the metal oxides, but function in a similar manner. Zeolyte membranes having pore sizes allowing diffusion of certain sized molecules across the membrane are also for specific ion conduction. Each type of metal oxide or membrane electrolyte finds use in a different ion conducting application.
The electrolyte elements are often formed as flat plates having an electrically conductive electrode material attached to a portion of one or both of the plate's flat surfaces. When the electrode covered electrolyte plates are exposed to a gas containing the uncharged form of the ionic species, an electrochemical reaction occurs between the electrode and the uncharged species to generate the specific ions.
Ion conducting capacity can be increased by arranging a plurality of electrolyte plates into a stack whereby each plate is separated from successive plates by electrically conductive spacers or interconnectors. The spacers allow reactant gases necessary for ion conducting activity to flow between the electrolyte plates, and come in contact with the electrode covered surfaces. The spacers may be grooved interconnectors as well known in the art, or a variety of other configurations offering particular advantages in certain applications. Grooved interconnectors typically have two grooved or channeled faces on opposing sides of the interconnector, with the grooves on one side being offset 90.degree. from those on the other. Other configurations for the spacers include internally manifolded interconnectors, and spacer bars or elements disposed between the ion conducting plates.
Ion conducting electrolytes, particularly metal oxide plates, find use in a variety of devices including fuel cells, steam electrolyzers, oxygen concentrators, and other types of electrochemical reactors. When used as a fuel cell, fuel gases such as H.sub.2, CH.sub.4 containing gases, synfuels, or light hydrocarbon fuel stocks, are directed to one face of the spaced apart electrolyte plates, and air is directed to the opposing face of the plates. When used as an oxygen concentrator, air is directed to one face of the plates, and pure molecular oxygen is collected from the other face. Other ion conducting devices using ion conducting electrolytes function in a similar manner, but may have structural modifications and different reactant gas requirements.
In a fuel cell, gases flowing between spaced apart electrolyte plates come in contact with, and react at, the electrode material attached to the surface of the plates. For example, at the electrode surface, an electrochemical reaction occurs in which an ionic species, such as O.sup.-2 from air, is produced and conducted across the plate to form water and CO.sub.2 on the opposite side. In the case of a typical fuel cell, this chemical reaction may be illustrated by the following equations:
air side: EQU 8e.sup.- +2O.sub.2 .fwdarw.4O.sup.-2
fuel side: EQU CH.sub.4 +4O.sup.-2 .fwdarw.CO.sub.2 +2H.sub.2 O+8e.sup.-
If the two electrodes on opposite sides of the plate are electrically connected, an electrical current may be obtained through passage of electrons between the two electrode surfaces. This reaction occurs when the plates reach an operating temperature, typically 600.degree.-1000.degree. C. for ceramic oxide based fuel cells, and lower for polymer and molten electrolyte membranes. Energy released from the electrochemical reactions contributes to maintaining the operating temperature. Conduction of the O.sup.-2 ions through the electrolyte plates occurs due to a difference in the partial pressure of O.sub.2 on opposite sides of the plates. In current based devices, an electrical potential applied across the plates can be used to drive the reaction.
Fuel gases and air are typically supplied to an ion conducting electrolyte stack by a manifold. A gas supply manifold may be attached to the side of the stack so that reactant gases are directed between the spaced apart electrolyte plates. Similarly, a gas collection manifold may be attached to the stack to collect gases produced during operation of the stack.
An increase in the output from an ion conducting device can be obtained by arranging several stacks together and electrically connecting the stacks in series or parallel. Parallel connection enables the device to continue functioning if one of the stacks fails, while series connection provides other advantages such as increased voltage output from fuel cells. Each stack must have access to reactant gases which necessitates use of a manifold. If the stacks are arranged in a block configuration whereby the stacks are positioned adjacent to each other, a manifold can be attached over all similarly oriented gas flow channels through the stacks. However this arrangement is subject to several problems. The flow of gases between the electrolyte plates becomes restricted as stacks are added to the assembly, resulting in inefficient cooling in the downstream stacks. Additionally, the downstream stacks may receive fuel which has been partially depleted of a reactive species. The close packing of the stacks, and reduced fuel and air flow also interferes with maintaining the assembly at a uniform temperature, further reducing the efficiency of the device. Additional stacks cannot be added to this type of manifold and stack arrangement without considerable difficulty, and construction of a new manifold.
Another option for supplying and collecting gases to and from the stacks is to manifold each stack individually. This option allows the stacks to be arranged in a pattern which facilitates uniform heating and gas flow, but makes for a bulky and difficult-to-manufacture device. The increased number of manifold seals also increases the probability of a leak occurring between the manifold and the stack. If a leak occurs in a fuel gas manifold, the fuel becomes diluted with air which severely reduces its potential to participate in the chemical reaction on the fuel side of the electrolyte plates.
Existing arrangements for the stacks of ion conducting electrolyte elements in solid state ion conducting devices also suffer from problems with oxidation of electrical pathway components. Because the electrical components are frequently exposed to the oxidizing environment of air, inexpensive electron conducting materials, such as nickel, cannot be used due to their propensity toward oxidation. This necessitates the use of expensive materials such as silver or platinum to avoid oxidation resulting in premature failure of the electrical pathway components.